In a color TV video signal the color subcarrier reference is transmitted during the horizontal blanking interval. An eight cycle burst of 3.58 MHz (approximately) reference is located subsequent to the synchronizing pulse on what is called the "back porch" of the blanking portion of the composite video waveform. This signal is used to phase lock a color subcarrier oscillator that provides the carrier used to demodulate the transmitted color signals. Desirably this burst signal is gated out of the composite video so that only the burst information is available for phase locking the subcarrier oscillator. It is important that the burst gate occurs during the horizontal blanking interval and that it be wide enough to embrace all of the burst gate interval yet narrow enough to exclude undesired signals. For example, if the burst gate overlaps into the video portion of the composite signal, color phase errors can develop to produce incorrect the response.
In the prior art the simplest approach has been to use the blanking pulse for color burst gating. This makes the circuit susceptible to any interference signal located inside the blanking interval. Therefore a narrower gating pulse is desired. Accordingly, numerous pulse shaping means have been employed to generate a suitable waveform. For example, in one approach the sync pulse is delayed and used as the gating signal. Alternatively, the trailing edge of the sync pulse is used to initiate a pulse generating circuit. One major problem associated with the above methods is that they start with transmitted video signals and are therefore subject to transmission noise and errors. It is generally preferred to employ the TV display flyback pulse as a blanking signal source. Since this source is independent of the signal channel, it provides a much cleaner signal less subject to noise and transmission errors. However, since the flyback pulse is almost as wide as the blanking pulse, it is not ideal. Accordingly, wave shaping is commonly employed and the resulting circuits have proven to be complicated and difficult to implement.